When you’re passing 48,000 or even 96,000 audio samples between digital devices every second, the exact timing is critical. The sync only has to be off a microscopic amount to create bad clicking noises that you really don’t want.
Instead, every device agrees to let one device be the clock master, and they all lock to its crystal clock instead of their own.
That’s why the Clock Source setting in iConfig’s Audio Info tab exists.
Normally you’d leave it set to Internal (the default) and all the computer devices lock to the interface. Most of the time you don’t even need to intervene.
But sometimes that’s not an option.
Consider the following setup, for example – an aggregate device with an iConnectAUDIO2+ and an iConnectMIDI4+. (Aggregate devices are audio interfaces that are combined on a Mac or PC. Software then sees one interface with the total number of ins and outs from both of them.)
Which interface should be the clock source? Given a choice, you’d generally make the interface doing the analog/digital or v.v. conversion the master. That would be the iConnectAUDIO2+, and that’s how we set it in Audio MIDI Setup, the Mac’s configuration program. Aggregate devices are created automatically in Windows – it has no configuration program, so you’re not given a choice.
Either way, you need to go to iConfig’s Audio Configuration tab and tell the iConnectMIDI4+ that it’s no longer boss. More specifically, you have to make the computer doing the aggregating the boss:
The iConnectAUDIO2+ can stay as is, on internal sync.
If you've never heard clicks and pops, well, good. That's normal, because the factory default setup is good for 95% of all setups. But if you have a setup with, say, aggregate devices on two computers and are having issues – again, you probably won’t! – then the best advice is to try the available clock schemes until you find the one that works.
By the way, iPads will just lock to the interface they’re connected to, so there’s no conundrum.
The only loose end is that comment about making the interface doing the analog-to-digital conversion the master. It doesn’t make a very big difference, so don’t worry if that’s not possible. The idea is that it’s easier for a device to lock to its own clock than to lock to another one, so it may sound better. Please do a search for “phase-lock loop” or "PLL circuit" if you want to get super-nerdy about this.
Congratulations. You are now a master of the Clock Source drop-down menu.